The present invention relates to a green-emitting phosphor suitable for high quality image display and an image display device using the same.
The image display device using cathode ray tubes, etc. to display color images is advancing toward higher density and higher luminance by recent ever-increasing needs, where the image display device means a device per se for displaying image information by emission caused by exciting a phosphor by electron beam irradiation or ultraviolet ray, for example, a cathode ray tube (particularly projection type cathode ray tube), a low acceleration electron beam display panel [field emitter display (FED) etc.] a plasma display panel (PDP), etc., and also a system for displaying images, using said tube or panel as a display part or upon further incorporation of a driving device, an image processing circuit, etc.
Due to the reduction in beam spot diameter and the increase in scanning speed and excitation intensity of an electron beam etc. to attain higher density and higher luminance, the image display device has suffered from considerable occurrence of after-image caused by luminance saturation, luminance degradation, and afterglow, and the resulting image degradation has been a problem. At the same time, color, emission with a good color reproducibility has been also required. Therefore, improvement in luminance saturation property, luminance degradation resistance, afterglow property and color emission of phosphors is now keenly desired.
Explanation will be made below mainly of projection type cathode ray tube, which will be hereinafter referred to as xe2x80x9cprojection tubexe2x80x9d, among images display devices. Luminance of a projection tube is controlled by a current level for generating excitation electron beam, and thus it is necessary that the luminance of the phosphor can be linearly increased with increasing current level, but generally the phosphor suffers from such a phenomenon as luminance saturation with increasing excitation intensities of electron beam, resulting in loss of luminance linearity. When images are displayed with higher excitation intensities, degradation of phosphor materials becomes so considerable that luminance and color emission will be degraded while used.
Luminance saturation property and luminance degradation resistance depend largely upon species of phosphor materials process, additives, composition, etc. Heretofore, selection of phosphors having better properties and improvement of composition and process have been made to improve these properties, but no single phosphor materials fully satisfying all the properties as required have been obtained yet.
Green-emitting phosphor occupies 70% luminance on a white screen, and thus its improvement is important. Materials so far used as green-emitting phosphor for the projection tube include, for example phosphors of Zn2SiO4:Mn series, Gd2O2S:Tb series, ZnS:Cu, Al series, Y2SiO5:Tb series, Y3 (Al, Ga)5O12:Tb series. etc.
Phosphors of Gd2O2S:Tb series with a good emission efficiency in the low-current level region have a low luminance in the high-current level region and cannot be used with an electron beam of high excitation intensity. Phosphors of Zn2SiO4:Mn series and ZnS:Cu, Al series have a high luminance saturation in the high-current level region, but when such phosphors are used in a projection tube, luminance of the projection tube will be lowered. Luminance degradation by electron beam takes place Vigorously and the life of the projection tube is considerably shorter. Phosphors of Y2SiO5:Tb series have a high luminance in the high-current level region, but a low luminance in the low-current level region, and luminance degradation by electron beam is considerable.
To improve these disadvantages, it is known to use phosphors of Y3 (Al, Ga)5O12; Tb series with a high luminance, a low luminance degradation and a low luminance saturation, but color emission of such phosphors is yellowish with a poor color reproducibility and thus such phosphor cannot be used alone in the image display device, as disclosed, for example, in JP-A-59-49279.
An object of the present invention is to improve color emission of green-emitting phosphors of Y3(Al, Ga)5O12:Tb series.
The present invention provides a green-emitting phosphor having a composition formula represented by (Y1-x-zGdxMa)3-3yTb3y (Al1-z, Gaz)5O12, where 0  less than xxe2x89xa61, 0xe2x89xa6a less than 1, 0 less than x+axe2x89xa61, 0 less than y less than 1 and 0xe2x89xa6zxe2x89xa61, and M is at least one of Sc, Yb and La.
Furthermore, the present invention provides an image display device using such a green-emitting phosphor.